题解 | 添加逗号

num=list(input())
length=len(num)
pos=0
if length <=3:
    print("".join(num))
else:
    yu=length%3
    if yu==0:
        pos=3
    else:
        pos=yu
    aa=len(num)
    while pos<len(num):
        num.insert(pos,",")
        pos+=4
    print("".join(num))